(Not Applicable)
(Not Applicable)
A basic transponder receives a signal from a SAR (synthetic-aperture radar), and then retransmits the signal back to the SAR. The problem with retransmitting the signal back to the SAR without any changes is that the signal must compete with ground reflection noise from natural and cultural ground clutter. If no type of modulation is used, then to stand out above the background noise, extra power and electronics are often needed to increase the gain in the return signal, often requiring two antennas, one for receiving and one for transmitting. Most transponders do not provide any added information in the retransmitted signal. In most cases the interrogator is a SAR located in an aircraft or satellite, and a transponder is located some distance away from the radar, normally on the ground.
The ability to provide command control and communications to and from a transponder has many potential applications. Besides the military applications for battlefield management, intelligence gathering and the like, there are commercial applications which include transponder status, other environmental status and emergency response. A RFID (radio frequency identification) system contains two main elements, an interrogator and one or more transponders. In the radar transmission between the interrogator and transponder, a RF signal is encoded within the SAR pulse to provide information between the interrogator and transponder, that is normally unknown to the other element.
U.S. Pat. No. 5,486,830 discloses a RFID system wherein digital codes are encoded in the SAR signal that is received by a transponder, also called a RF tag or simply tag. A single antenna can be utilized by this transponder to transmit and receive signals. This is accomplished by a time-gating method using a 50% duty cycle factor for setting the tag""s transmitting and receiving intervals, which are mutually exclusive. The received SAR signal is mixed with a reference oscillator to provide a detected signal that can be decoded. Tag logic and timing circuits measure the time between detected pulses and decode these pulses into downlink commands from the SAR, symbols of the downlink commands are encoded in the spacing between the SAR pulses. Downlink commands contain mode information that allows the SAR and tag to obtain a common pulse index (coarse synchronization). However, in order to achieve fine synchronization the tag must average the measured time-of-arrival of a number of pulses. After fine synchronization is achieved, and if so commanded, the tag will go into the uplink mode. The tag device phase encodes its echo with a sequence containing both prescribed and periodic or pseudo-random patterns, containing status or information unknown to the radar source""s signal processor. A bi-phase (0/pi) modulator is utilized to allow selective amplification of +1 or xe2x88x921 of the signal before retransmitting the signal back to the SAR. The signal processing at the SAR mixes a sequence identical to the prescribed periodic or pseudo-random selective amplification against the received echoes. This selective amplification spreads the spectrum of the natural or cultural echoes and de-spreads the tag""s echo, making the retransmitted signal stand cut above the natural or cultural noise.
The present invention is an improvement over the other RFID systems. Prior art used simple pulses defined by their spacings in the SAR pulses. The pulses represent symbols that encode the digital information contained in the SAR pulses.
The present invention uses SAR LFM (linear frequency modulation) pulse waveforms. One advantage of using LFM pulse waveforms is that these waveforms are not noise sensitive. While the symbols of the encoded digital code are determined by the time internal between the pulses as in U.S. Pat. No. 5,486,830, noise can cause an incorrect interval to be detected and thereby generate an invalid message.
Two receivers, an AM and a FM receiver, are connected to an antenna and are used to demodulate their respective components of the LFM pulse waveform. The output of the AM receiver is the demodulated envelope that is proportional to the amplitude of the AM component of the LFM pulse waveform. The amplitude and time duration are compared to a preprogrammed threshold and time duration criteria. The preprogrammed criteria are stored in the tag DSP (Digital Signal Processor. If both the threshold value and time duration are determined to be valid, then the demodulated FM portion of the LFM pulse waveform is checked for validity.
A reference local oscillator is needed to demodulate FM signal component of a LFM pulse waveform. This is generated by delaying LFM pulse waveform and mixing it with the non-delayed LFM pulse waveform. A separate reference oscillator is, therefore, not required. The demodulated waveform is then passed through a zero-crossing detector. The output of the zero-crossing detector is sampled and the samples are counted by the DSP to estimate the average frequency over the pulse duration.
In order to determine the slope of the frequency deviation, a 90xc2x0 power splitter is added to the local oscillator, before the mixer. The power splitter has two outputs, one in-phase and one in quadrature-phase with the LFM pulse waveform. The signal component set of mixer, lowpass filter, zero-crossing detector and sampler is replaced with two identical sets of components. The output of the set with the in-phase signal is labeled xe2x80x9cFM In-Phasexe2x80x9d, and the output of the set with the quadrature-phase signal is labeled xe2x80x9cFM quadrature-phasexe2x80x9d. These two outputs are then used by the DSP to determine the slope of the frequency deviation.
The DSP utilizes the slope information and frequency deviation to determine the message sent by the SAR. Though the use of Phase Modulation (PM), the tag can encode data to send back to the SAR.
By using RF switches, the signals can be directed so that the same antenna can be used for receiving and transmitting. Since the same antenna is used, a chop timing of the RF switches in required. There are 3 stages in the chop signal: receive, transmit and blanking. The blanking stage is required to prevent oscillations in the tag due to reflections from nearby objects. Some systems require randomizing the blanking time to prevent the radar from accidentally locking onto the spectral lines that are generated during the chopping of the RF signal. The blanking times can change pseudo-randomly each cycle.